A bipolar plate is the backbone of an ionomer membrane fuel cell stack or battery. An ionomer membrane is virtually any ion-conducting membrane. The most technically advanced type of ion-conducting membrane currently available for fuel cell applications is the proton-exchange membrane, such as the Nafion series of membranes, the Dow membrane, etc. The fuel cell electrodes are hot-pressed or otherwise affixed to the membrane to form a unitized assembly. Bipolar plates, and associated gas seals, enclose the membrane and electrode assembly ("MEA") in a fuel cell.
Typical state-of-the-art bipolar plates are made of graphite that is compressed into a single block. Gas flow channels (the "flow-field" channels) are generally machined into the graphite block and permit the flow of the reactant gases from the manifolds and through the flow-field to the electrodes of the fuel cell. Bipolar plates serve three primary functions in overall fuel cell operation. First, they conduct electricity from the fuel side of the electrochemical reaction to the oxidant side of the reaction, where water is produced. Second, they separate the fuel and oxidant gases and prevent cross-mixing of the reactant gases in the cell. Third, they allow gases from the manifolds to reach the appropriate fuel cell electrode. The gas seals or gaskets (the "gaskets") serve to contain the gases within the fuel cell and also prevent cross-mixing of the reactant gases.
Graphite is an excellent material for use in fuel cell applications because it is relatively inert in the corrosive electrochemical environment of the cell. Although the material cost of graphite is not high, the manufacturing methods currently employed result in very costly bipolar plates. Also, because state-of-the-art bipolar graphite plates are compressed into a block, they tend to be relatively thick. A relatively thick plate is also required in order to accommodate the channels of the flow-field. Separate cooling plates are often included in fuel cell designs, which may further add thickness to the fuel cell stack.
As the thickness of the graphite bipolar plates increase, the number of cells that can be placed in a given spatial volume decreases. For example, some state-of-the-art ionomer membrane fuel cells, utilizing a standard machined graphite bipolar plate, may be approximately 100 mils (ca. 2.5 mm) or more thick. Up to ten cells can therefore be stacked per lineal inch of fuel cell stack using these types of cells. If the thickness of the bipolar plate could be reduced, however, much thinner fuel cells could be produced and the cell "stacking density" (i.e., the number of cells in a given volumetric space) could be correspondingly increased. An increase in stacking density would be particularly beneficial in portable and transportation-related applications where more compact and light-weight fuel cell stacks and fuel cell batteries are desirable.
While some work has recently been done in reducing the thickness of the bipolar plate and increasing stacking density, these efforts have focused primarily on the substitution of a metal plate, such as a plate made of titanium, for the traditional graphite bipolar plate. For example, Lynntech, Inc. of College Station, Texas, has reported that its titanium/foamed metal bipolar plate allows stacking densities of up to 5.5 cells per centimeter, or nearly 14 cells per inch. H Power of Belleville, N. J. has developed a bipolar "platelet", also made of titanium, which permits about 12 cells per inch to be stacked. Dr. Mahlon Wilson of Los Alamos National Laboratories has developed a stainless steel screen bipolar plate that allows stacking of about 10 cells per inch.